Acoustic resonators can be used to implement signal processing functions in various electronic applications. For example, some cellular phones and other communication devices use acoustic resonators to implement frequency filters for transmitted and/or received signals. Several different types of acoustic resonators can be used according to different applications, with examples including bulk acoustic wave (BAW) resonators such as thin film bulk acoustic resonators (FBARs), coupled resonator filters (CRFs), double bulk acoustic resonators (DBARs), and solidly mounted resonators (SMRs), to name but a few.
FIG. 1 is a block diagram illustrating an example of a band pass filter 100 formed by a plurality of acoustic resonators. Band pass filter 100 has a ladder circuit configuration that can be found, for instance in duplexer circuits associated with transmitters and receivers of mobile telephones.
Referring to FIG. 1, band pass filter 100 comprises a plurality of series resonators 105 and a plurality of shunt resonators 110 connected between an input port and an output port. Series resonators 105 have higher resonant frequencies than shunt resonators 110. Accordingly, they allow higher frequencies to pass through while shunting out lower frequencies.
FIG. 2 is a cross-sectional view illustrating an acoustic resonator 200 that can be included as one of series resonators 105 or shunt resonators 110 in the example of FIG. 1. In this example, BAW resonator 200 is an FBAR.
Referring to FIG. 2, acoustic resonator 200 comprises a substrate 205 and an acoustic stack 210 formed on substrate 205. Acoustic stack 210 comprises a piezoelectric layer 220 disposed between a first electrode 215 and a second electrode 225. Piezoelectric layer 220 comprises a piezoelectric material that converts electrical energy into mechanical movement and vice versa.
During typical operation, an electrical bias applied between first electrode 215 and second electrode 225 causes piezoelectric layer 220 to expand (or contract, depending on a phase of electrical signal) through the inverse piezoelectric effect. The expansion (or contraction) of piezoelectric layer 220 produces electric charge through the direct piezoelectric effect, which is then presented to the electrodes. Where the frequency of the electrical signal and the natural mechanical resonance frequency of acoustic stack 210 are close to each other, an electromechanical resonant state occurs resulting in significant mechanical displacements of particles comprising acoustic stack 210 and significant modification of electrical signal at electrodes 215 and 225. This electrical response is a basis of signal filtering in band pass filter 100 of FIG. 1. Where the frequency of the electrical signal is far away from mechanical resonance frequency of the stack 210, the mechanical displacement of particles is negligible and so is the produced charge, thus resulting in a standard capacitor-like electrical response of the resonator 200.
An air cavity 230 is formed in substrate 205 to facilitate mechanical movement of acoustic stack 210. Air cavity 230 facilitates mechanical movement by creating acoustic isolation between acoustic stack 210 and substrate 205. This acoustic isolation prevents acoustic stack 210 from losing mechanical energy to substrate 230, which in turn prevents acoustic stack 210 from losing signal strength.
A microcap 235 is connected to acoustic stack 210 using wafer bonding technology. Microcap 235 can be formed, for instance, by etching a cavity in a silicon wafer and placing the cavity over acoustic stack 210. It can also be formed, for instance, by attaching an annular gasket to substrate 205 and placing silicon over the annular gasket. Microcap 235 forms an air cavity 245 over acoustic stack 210 and allows for unobstructed movement of acoustic stack 210. It also hermetically seals acoustic resonator 200 to prevent damage from environmental factors such as humidity.
In addition to the features shown in FIG. 2, acoustic resonator 200 typically comprises electrical contact pads connected to first and second electrodes 215 and 225. The electrical contact pads extend outward from the sides of acoustic resonator 200 to transmit input and output signals to first and second electrodes 215 and 225, respectively.
Significant design considerations for acoustic resonators include, among other things, cost, chip area, and response characteristics. There are various factors that affect each of these considerations. For instance, the cost of a resonator typically varies according to the materials and processes used in its manufacture. The chip area, meanwhile, tends to vary according to the lateral width of the acoustic stack and associated components, such as the microcap and contact pads. In general, the lateral width of the acoustic stack varies according to its passband, with lower frequency resonators occupying more space than higher frequency resonators. The response characteristics of an acoustic resonator are defined by various parameters, such as an electromechanical coupling coefficient (kt2) and a quality (Q) factor. The electromechanical coupling coefficient kt2 indicates the efficiency of energy transfer between electrodes and the piezoelectric materials. This coefficient influences insertion loss and bandwidth of the filter 100. The Q factor affects roll-off of the filter 100, and it varies according to various material properties of the acoustic resonator 200, such as a series resistance Rs and a parallel resistance Rp, which correspond to various heat losses and acoustic losses of the resonator 200.